This invention relates to a knocking detecting apparatus for an internal combustion engine which enables to perform an accurate detection of trace knock through provision of a clamp circuit.
In a conventional knocking detecting apparatus shown in FIG. 1, a signal 5 which is detected by a knocking detector (knock sensor) 1 passes through an input circuit 2 including a filter etc. and becomes the one input signal 7 of a comparator circuit 4, while at the same time it is applied to a discrimination level generator circuit 3 which generates a knocking discrimination level 6 as the other input of the comparator 4. When the signal 7 is larger than the signal 6, the comparator 4 generates a knock signal 8.
However, in such a prior art constitution as described above, the input signal 7 of the comparator 4 increases in accordance with the increase of the number of revolutions N of the engine, as shown by numerals 71, 72 and 73 in FIG. 3, where 71 denotes a noise level in the absence of knocking, 72 denotes a trace knock level, and 73 denotes a light knock level. We define here a trace knock as a knock state which can be barely heard by human ear, while a light knock as a knock state which can be clearly heard by human ear. The knocking discrimination level 6 increases also in accordance with the increase of the number of revolutions N of the engine, as shown by 61 in FIG. 3. A poor adjustment of tappet clearance may result in valve-down, etc., and noise due to the valve-down, that is, the noise which may occur by the closure of intake and exhaust valve of a cylinder which is other than the cylinder in which the ignition occurs, and other noises lead to an increase in the noise level. As a result, it may occur that in the high speed rotation region of the engine the knocking discrimination level 6 exceeds the trace knock level as shown by a curve 1 - 1 '. In such a case a problem that generation of trace knock can not be detected may occur. Furthermore, it may occur that operational amplifiers used in the filter of input circuit 2 and operational amplifiers used in the amplifier of knocking discrimination level generator circuit 3 are of different kind and hence have different saturation voltages, or that they are of the same kind but have variation in their saturation voltages and the filter saturates at a lower voltage. In such cases, if the sensitivity of the knock sensor 1 is extremely large, the signal 7 having passed through the filter of the input circuit 2 may saturate at a voltage lower than the knocking discrimination level 6 denoted by 61 in FIG. 4, as shown by the trace knock level 72 in FIG. 4, and this causes a problem that the knocking detection cannot be made.